A telescope having a variable magnification is known from DE 704 17 03, which is incorporated herein by reference. The telescope has an optical system in the form of a zoom objective, a prism reversal system, and an ocular. The zoom objective is provided with four lens units, namely a first lens unit, a second lens unit, a third lens unit, and a fourth lens unit. In other words, the zoom objective is a four-number system. Two of the above-mentioned lens units, namely the second lens unit and the third lens unit, are situated so they are movable along the optical axis. In contrast, the first lens unit and the fourth lens unit are situated fixed on the optical axis. The fourth lens unit is used for implementing the main portion of the required refractive power of the objective. In principle, the fourth lens unit is a main objective, essentially having an afocal adapter having variable magnification connected upstream. The afocal adapter includes the fixed first lens unit, the movable second lens unit, and the movable third lens unit. However, the above-mentioned construction, including a main objective having an afocal adapter, has the disadvantage that a large overall objective length results in comparison to the maximum achievable focal length of the zoom objective. This is undesirable.
A telescope having an objective of variable focal length (zoom objective), a prism reversal system, and having an ocular having a fixed focal length is known from U.S. Pat. No. 3,069,972, which is incorporated herein by reference. The objective of the known telescope is constructed according to the principle of optical compensation. For this reason, only a movement along the optical axis is necessary, which is executed by a first lens unit and a second lens unit, between which a fixed third lens unit is situated. The known telescope, however, has the disadvantage because of the principle of optical compensation that the location of the image plane of the object to be imaged fluctuates over the magnification range of the telescope.
A zoom telescope is known from DE 10 2004 001 481 A1, which is incorporated herein by reference, which includes an optical system in the form of an objective of variable focal length, an ocular of fixed focal length, and a prism reversal system, the prism reversal system being situated between the objective and the ocular. The objective has a first lens unit, a second lens unit, and a third lens unit from an object in the direction of the ocular. The second lens unit and the third lens unit are situated so they are movable along the optical axis of the objective for zooming. The first lens unit of the objective includes a first lens group, which has a cemented element, and a second lens group, which only includes a single lens. If the location of the image plane of the object is to remain unchanged during zooming when finite object distances are set, focusing must be performed by moving lens units along the optical axis of the objective, these movable lens units being in front of the movable second lens unit and the movable third lens unit, which are used for changing the focal length. It is known from DE 10 2004 001 481 A1 that the setting to finite object distance is performed by displacing the entire first lens unit along the optical axis of the objective or, alternatively thereto, only by a movement of the second lens group of the first lens unit, the second lens group including a single lens. A constant field of vision is achieved by using a zoom objective and by situating a field aperture in the intermediate image after the zoom objective and/or the prism reversal system. An intermediate image having equal diameter results for all zoom settings of the zoom objective in this way. A constant subjective field of vision results after the ocular, independently of the zoom setting. However, the known telescope has the disadvantage that a displacement of the entire first lens unit to set finite object distances requires an overall length change of the objective (and thus also of the entire telescope). This can be avoided using internal focusing by sole movement of the second lens group of the first lens unit. However, because only a single lens is displaced for setting to finite object distance, the spherical aberration and the chromatic errors of the objective change very strongly. This is disadvantageous in particular for objectives having a large maximum focal length (for example, 500 mm).
An objective of an above-mentioned telescope from the prior art is typically achromatically corrected. The vertex image distances for red and blue light are thus equally long, but the vertex image distances for green light deviate. This deviation is typically referred to as a longitudinal chromatic aberration or also as a secondary spectrum. The secondary spectrum of the objective of a telescope of this type often results in color fringes at light-dark transitions. These are often very annoying to an observer who observes an object using the telescope. This interference is all the more obvious the greater the focal length of the objective and thus a magnification of the object is selected on the telescope.
An achromatic lens system is known from U.S. Pat. No. 6,226,132 B1, which is incorporated herein by reference, in which the secondary spectrum may be decreased by selecting a suitable deviating relative partial dispersion. However, the relative aperture of this known lens system is 1:10. Such a relative aperture is inadequate for use in objectives having variable focal lengths for telescopes.
Accordingly, it would be desirable to provide an optical system in which both the spherical aberration and also the Gaussian error are corrected sufficiently and in which the overall length does not change.